NOE-experiments
A variety of nOe experiments exist. The most important ones are:
1D- steady-state nOE experiments:
These experiments generate equilibrium population of one spin by continuous irradiation of one (separated) resonance, similar to the presaturation on water. The implest form is a standard 1D experiment, in which a signal intensity change uponselective irradiation can be observed. Usually more useful is the acquisition of a difference spectrum: a 1D spectrum with irradiation is coadded to a reference spectrum without irradiation. This yieds a strong negative eak at the irradiation frequency, and difference peaks for all resonances which experience an nOe effect, while lines without nOe cancel out.
The 2D-NOESY experiment is the most useful experiments, as all nOe effect between spins can be measured simultaneously. The nOe is generated by simultaneous, temporary population changes, which is called “transient” nOe, not a selective inversion or steady-state irradiation. The nOe gives rise to magnetization transfer and off-diagonal peaks, connecting the resonance frequencies of the spins which couple/ cross relax through space.
Notes:
You might note at this point, that this is yet another pulse sequence with three 90 degree pulses. Again, the specific phase cycling is what distinguishes this experiment from previous ones (such as the DQF-COSY), as the phase cycling selects for the desired coherences, and (ideally) supresses all undesired ones, as well as artifacts due to the relaxation of spins during the pulse sequence (axial peak supression).
As we have already explained the phenomenon of the nOe on the basis of population changes and subsequent relaxation events, and the noesy-experiment does not involve any higher order coherences (at least for the observed signals) we can explain the spin behaviour in the vector model.